The present invention relates to an insert which is to be interposed between two base metal bodies for achieving liquid phase diffusion bonding of these base metal bodies.
In order to join, the base metal bodies of such materials as high-alloy which are difficult to apply fusion-weld techniques, or in order to join bodies of base metals which have large joining interface, the method called "diffusion bonding" is employed. This method consists in first holding two base metal bodies to be bonded together by proper means and then heating them in vacuum or an inert-gas atmosphere, thereby diffusing metal body atoms from one base metal into the other, or from an insert to one or both base metal bodies, and thus bonding the base metal bodies together without melting the base metals. Various methods of diffusion bondings are known. They are classified into three types, direct diffusion bonding, solid phase diffusion bonding, and liquid phase diffusion bonding.
In direct diffusion bonding, two base metal bodies are held in face-to-face contact. In solid phase diffusion bonding, an insert is interposed between two base metal bodies and metal atoms diffuse from the insert into the base metals without melting the insert. In liquid phase diffusion bonding, an insert is interposed between two base metals and is melted, whereby metal atoms diffuse from the insert into the base metal bodies. Liquid phase diffusion bonding is advantageous over the other two diffusion bonding methods in that a lower pressure is required to hold the base metal bodies and/or the insert together.
The insert used in the liquid phase diffusion bonding is made of a metal or an alloy having a melting point lower than that of the base metal bodies. The assembly consisting of the base metal bodies and the insert interposed between the base metals is heated in vacuum or an inertgas atmosphere to the melting point of the insert or a higher temperature, thereby melting the insert. As the insert melts, the metal atoms diffuse from the insert into the base metal bodies, whereby the base metal bodies are bonded. (See U.S. Pat. No. 3,678,570 to Paulonis et al.) It is desirable that the insert be thin enough, 5 to 200 .mu.m, to allow the metal atoms to diffuse easily into the base metal bodies. Two types of such thin inserts are available. The first type is a film made of powder of insert material or a foil of insert material. The second is an insert made of one or two layers of insert material formed on the joining surfaces of the base metal bodies, respectively, either by plating or by spraying.
When the insert used in the liquid phase diffusion bonding is a film made of powder of insert material or consists of one or two layers of insert material formed on the base metal bodies by spraying, respectively, an oxide is likely to be formed on the joining surfaces of the base metal bodies. Further, since it is difficult to make such an insert have a uniform thickness, some portions of the insert will remain on the joining surfaces of the base metal bodies after the base metal bodies have been heated. For these reasons, the bonding strength attained is insufficient. On the other hand, the insert, which consists of one or two layers of insert material plated on the base metal bodies, can have a uniform thickness if the base metal bodies have simple shapes. If these base metals are large, or have complicated shapes, the insert can hardly have a uniform thickness.
On the other hand, it is relatively easy to form a foil of insert material, having a uniform thickness. This foil can therefore provide an insert layer having a uniform thickness for large joining surfaces. The foil can be made either by rolling or by rapid solidification process. One of the alloys that can be rolled into a foil having 5 to 200 .mu.m is Al-13% Si alloy. This alloy is usually used in component of a so-called brazing sheet. The brazing sheet is made first by sandwiching a thin aluminum plate between two thin plates of Al-13% Si alloy and then by rolling all these plates together. As is disclosed in Japanese Patent Disclosure No. 119683 (1981) KOKAI, the brazing sheet is interposed between a Ti plate and an Al plate, or between two Al plates, thus to join these plates by liquid phase diffusion bonding.
To fasten two base metal bodies of Ni-based alloy by liquid phase diffusion bonding, an insert made of a Ni-based alloy containing P, B or Si which lowers the melting point of the Ni-based alloy, is often used. This alloy and the other low melting-point alloys forming inserts for liquid phase diffusion bonding can hardly be rolled into foils having a thickness of 5 to 200 .mu.m. This is because all these alloys have a small workability and the foils made by hot-rolling these metals are apt to have cracks. Indeed a rapid solidification process can provide thin foils made of these alloys and having a thickness of 5 to 200 .mu.m, but cannot provide thin foils having a size of 1 m (length).times.0.5 m (wide) or a wider size. Most of the low melting-point alloys, generally used as the materials of inserts for liquid phase diffusion bonding, are brittle. A thin foil made of any of these alloys and interposed between two base metals will be apt to be torn when the base metals slide against each other. In some cases, the thin foil can be torn while it is inserted into the gap between the base metals.